The present invention is directed to an improved process for making metal castings using a lost foam casting process.
Lost Foam Casting (Full Mold Casting) involves placing a plastic pattern of the desired cast part in sand and then pouring molten metal onto the plastic casting causing it to vaporize. The molten metal exactly reproduces the plastic pattern to provide the ultimate casting. Many patents have issued covering the Lost Foam Casting process.
It is known that polystyrene, the major polymer used in this application, produces surface defects when casting iron due to carbon residues left by the polymer. When casting low carbon steel the carbon formed from the polystyrene dissolves in the metal degrading the properties of the cast part. A number of patents describe variations in the Lost Foam Casting process that are meant to minimize the residues left by the polymer after the metal has been poured. Most of these variations involve changing the coating on the pattern or changing the flask in which the casting is made. For example, U.S. Pat. Nos. 4,448,235 and 4,482,000 describe a variable-permeability coating designed to avoid entrapment of polymer vapors in the casting. U.S. Pat. No. 3,572,421 describes a flask containing many air breathing holes to allow the escape of polymer degradation products to decrease the formation of carbon. Similarly, U.S. Pat. Nos. 3,842,899, 3,861,447, and 4,612,968 describe the addition of vacuum to the casting flask to aid in the removal of the polymer residues.
The Dow Chemical Company has recently reported the development of a polymethyl methacrylate foam bead useful to replace polystyrene for the Lost Foam Casting process. (Moll and Johnson, "Eliminate the Lustrous Carbon Defect with New Moldable Foam", Evaporative Foam Casting Technology II Conference, Nov. 12-13, 1986, Rosemont, Ill.). Although this polymer reduces residues left on the cast part, it carries with it other disadvantages. The higher glass transition temperature (130.degree. C.) of the polymer causes longer molding cycles when preparing patterns. It uses a Freon blowing agent which has been shown to cause corrosion of molds. It also rapidly gives off a large volume of toxic gas when castings are made. It is very difficult to control the evolution of gas and often the molten metal is blown back out of the flask.
There is still a great need for a polymer that provides the advantages of polystyrene but produces no carbon defects. U.S. Pat. No. 4,633,929 addresses this need with a polyalkylene carbonate copolymer which decomposes cleanly to give no carbon defects. The synthesis of various polyalkylene carbonate copolymers is well known in the art (see Inoue, U.S. Pat. No. 3,900,424). It is also well established that polyalkylene carbonates readily undergo thermal decomposition with little or no carbon residue (Frechet, Macromolecules 19, 13-19 (1986); and Polymer J. 19, 31-49 (1987)). These facts were applied to Lost Foam Casting by U.S. Pat. No. 4,633,929.
U.S. Pat. No. 4,633,929 teaches to use polyethylene carbonate and polypropylene carbonate in patterns for a foam process. However, the glass transition temperatures of these two materials are 10.degree. C. and 42.degree. C., respectively, and the temperatures at which the sand is recycled in the Lost Foam Process is normally between 65.degree. and 80.degree. C. Further, the ceramic coating on the pattern is dried at 65.degree.-80.degree. C. When subjected to these temperatures a polymer with a Tg of less than 80.degree. C. would tend to deform.
The known freeze-drying foaming method for making foam patterns gives polyalkylene carbonate with high density, nonuniform cell structure and poor surface quality. Complex patterns are difficult to make using this approach. Foam sheet can be made using this approach but cutting patterns from foam sheet is expensive and not favored by manufacturers using the Lost Foam Casting process. For utility, the polyalkylene carbonate should be in the form of small spherical beads which can be easily expanded and molded to provide a foam pattern with 0.9-1.3 pcf density.
Little prior art is available on preparation of foam from polycarbonate polymers. U.S. Pat. No. 4,587,272 shows foams prepared from aromatic polycarbonates using chemical blowing agents. However, there is no prior art for preparing low density polyalkylene carbonate foam patterns suitable for Lost Foam Casting.